The semiconductor device fabrication process uses plasma processing at different stages of the fabrication process. Plasma processing involves energizing a gas mixture by imparting energy to the gas molecules by introducing RF (radio frequency) energy into the gas mixture. This gas mixture is typically contained in a vacuum chamber, referred to as a plasma chamber, and the RF energy is typically introduced into the plasma chamber through electrodes.
In a typical plasma process, the RF generator generates power at a radio frequency—which is broadly understood as being within the range of 3 kHz and 300 GHz—and this power is transmitted through RF cables and networks to the plasma chamber. In order to provide efficient transfer of power from the RF generator to the plasma chamber, an intermediary circuit is used to match the fixed impedance of the RF generator with the variable impedance of the plasma chamber. Such an intermediary circuit is commonly referred to as an RF impedance matching network, or more simply as an RF matching network.
The purpose of the RF matching network is to transform the variable plasma impedance to a value that more closely matches the fixed impedance of the RF generator. In many cases, particularly in semiconductor fabrication processes, the system impedance of the RF generator is fixed at 50 Ohms, and RF power is transmitted through coaxial cables which also have a fixed impedance of 50 Ohms. Unlike the impedance of the RF generator and the coaxial cables, the impedance of the plasma, which is driven by the RF power, varies. In order to effectively transmit RF power from the RF generator and the coaxial cables to the plasma chamber, the impedance of the plasma chamber must be transformed to non-reactive 50 Ohms (i.e., 50+j0). Doing so will help maximize the amount of RF power transmitted into the plasma chamber.
The typical RF matching network includes variable capacitors and a control circuit with a microprocessor to control the capacitance values of the variable capacitors. The value and size of the variable capacitors within the RF matching network are determined by the power handling capability, frequency of operation, and impedance range of the plasma chamber.
The predominant type of variable capacitor used in RF matching network applications is a Vacuum Variable Capacitor (VVC). The VVC is an electromechanical device, having two concentric metallic rings that are moved in relation to each other to change capacitance. In complex semiconductor fabrication processes using plasma chambers, where the impedance changes are often frequent, the frequent adjustments needing to be made to a VVC leads to mechanical failures, often within less than a year of use for individual VVCs. Failure of a VVC leads to downtime for fabrication equipment so that the failed VVC can be replaced. Due to a desire to eliminate points of mechanical failure in the semiconductor fabrication process, it is unsurprising that the VVCs in RF matching networks are one of the last electromechanical components that remain in wide use in the semiconductor fabrication process.
As semiconductor devices shrink in size and become more complex, the feature geometries become very small. As a result, the processing time for each individual step needed to fabricate these small features has likewise been reduced—typically in the range of 5-6 s. RF matching networks which use VVCs generally take in the range of 1-2 s to match the plasma chamber impedance to the RF generator impedance. During a significant amount of the matching process, which includes the microprocessor determining the capacitances for the VVCs needed to create the match, controlling the VVCs to the achieve the determined capacitances, and then finally time for the RF matching network circuits to stabilize with the new capacitances, the fabrication process parameters are unstable, and these unstable process parameters must be accounted for as part of the overall fabrication process. Because the matching process time is becoming a more and more significant part of the time for each fabrication process step, the period in which process parameters are unstable becomes more of a factor in the overall fabrication process.
While Electronically Variable Capacitor (EVC) technology is known (see U.S. Pat. No. 7,251,121, the disclosure of which is incorporated herein by reference in its entirety), it has yet to be developed into an industry-accepted replacement for VVCs. However, because an EVC is purely an electronic device, an EVC is not a one-for-one replacement for a VVC in an RF matching network. Further advancements are therefore needed to more fully take advantage of using EVCs as part of an RF matching network.
For example, further advancements are needed in determining the capacitances necessary for an impedance match. A typical RF matching network based on VVCs uses information gathered from a power sensor to determine whether it has matched the input impedance to the desired impedance (e.g., 50 Ohms) or not. The power sensor can be a phase/magnitude detector, a directional coupler, or a voltage/current sensor.
In the case of a phase/magnitude detector, the detector is set such that when the input impedance is tuned to the desired impedance (e.g., 50 Ohms) the error signal out of the phase/magnitude detector goes to a minimum. In this case, the control circuitry of the RF matching network is designed such that it moves the VVC capacitors to bring the error signals out of the phase/magnitude detector to minimum. Once that state is reached, the RF matching network is considered tuned.
In the case of a directional coupler, the coupler is set such that when the reflected power is minimum, its reflected port shows a minimum signal. In this case, the control circuitry of the RF matching network is designed such that it moves the VVC capacitors to bring the reflected port signal to a minimum. Once that state is reached, the RF matching network is considered tuned.
The case of a voltage/current sensor is similar to a directional coupler. In this case, the voltage and current signals along with the phase angle information between the voltage and current signals is used by the control circuitry to first calculate the impedances and then the reflected power or reflection coefficient or simply the reflected power and/or the reflection coefficient. In this case, the control circuitry of the RF matching network is designed such that it moves the VVC capacitors to bring the calculated reflected power or the calculated reflection coefficient to a minimum. Once that state is reached, the RF matching network is considered tuned. These approaches, however, are time consuming in an industry where speed is of increasing value.
Further, there is need for improved control of the capacitance provided by the EVC, which in turn provides improved control of impedance matching.